1. Field of the Invention
The present invention relates to an apparatus and method for testing the performance of a communication receiver, and more particularly, to an apparatus and method for verifying the performance of a digital portion of an RF receiver.
While the invention is subject to a wide range of receiver applications, it is especially suited for testing the performance of a digital receiver for a satellite communication system capable of receiving quadrature phase shift keyed signals for voice and data, and is described in that connection.
2. Description of Related Art
In a typical satellite communication system, incoming signals from a conventional wired telephone are conducted from the public switch telephone network to a satellite ground station, which in turn transmits RF signals to the satellite for retransmission to a mobile terminal unit. Presently, a satellite system that covers large geographical areas typically use several satellites that follow different paths at low or medium altitudes so that at least one satellite is at all times covering the desired geographical area. From the standpoint of receiving signals, the low and medium altitude satellites have the advantage of being able to transmit a signal that reaches a mobile terminal unit at the earth's surface with a relatively large amplitude and without appreciable fading. It has been proposed, however, to provide a satellite communications network that utilizes a high altitude geosynchronous satellite which is capable of covering an area corresponding to a substantial portion of the North American Continent, so that a total of approximately six satellite beams will cover the entire Continent from Alaska to Mexico. The satellite for such a network will be approximately 22,600 miles above the equator and will be designed to operate in the L-band of RF frequencies. Energy traveling this great distance undergoes huge attenuation such that the power flux density incident at the antenna of the mobile unit is approximately 10.sup.-14 watts per square meter. This grossly attenuated signal is further degraded by background noise, and other satellite channel impairments such as Rician fading, gaussian noise, phase noise, and co-channel and adjacent channel interference.
Mobile terminal units capable of receiving these highly attenuated and impaired signals have been developed. One type of such mobile terminal includes a satellite receiver that has analog, digital and software subsections. In order to ensure that the digital and software subsections are capable of working as intended, it is of course advantageous to test such sections prior to completing assembly of the unit. However, in order to insure that the digital and software positions are defect free, it is necessary to subject the input of the digital processing board to digital signals corresponding to signals that the RF section of the receivers would receive including signals that would be received under adverse conditions.
To the knowledge of the inventors, no conventional apparatuses is in verify the proper operation of a digital processor board of a digital receiver.
In light of the foregoing, there is a need for an apparatus and method of verifying the performance of the processing board of a digital receiver in response to digital signals that correspond to signals having all of the possible interferences and impairments received from the satellite of a system in actual operation.